Fix "undefined reference to `fmt::v7::detail::basic_data<void>::digits'"

[phprus]

If static fmt library is used in shared library.
Error in commit 13e6529.

[vitaut]

I wonder why it doesn't happen in CI. Could you provide repro details?

[phprus]

Pseudocmake:

add_library(mylibrary SHARED lib.cpp)
target_link_libraries(mylibrary PRIVATE libfmt.a) # link with static fmt

add_executable(exe main.cpp)
target_link_libraries(exe PRIVATE mylibrary) # Error: undefined reference to `fmt::v7::detail::basic_data<void>::digits'

Because extern template struct basic_data<void>; and template struct detail::basic_data<void>; with visibility attribute is add reference to fmt::v7::detail::basic_data<void>::digits into mylibrary.so.

If link exe with libfmt.a, there will be no error.

[sergiud]

Adding the combination to the test suite would certainly help catch problems in the future.

[sergiud]

I believe, you are also linking to fmtlib incorrectly because the error occurs not in mylibrary but according to your description first in the exe target.

Note that libfmt is not linked transitively to exe because you specify the PRIVATE modifier:

add_library(mylibrary SHARED lib.cpp)
target_link_libraries(mylibrary PRIVATE libfmt.a) # link with static fmt

while you seem to be using libfmt in your exe target as well:

add_executable(exe main.cpp)
target_link_libraries(exe PRIVATE mylibrary) # Error: undefined reference to `fmt::v7::detail::basic_data<void>::digits'

I think, a clear (non-pseudo) example is necessary to understand the scenario.

[phprus]

@sergiud, No.
Now, mylibrary has are reference to fmt::v7::detail::basic_data<void>::digits always. Any use of mylibrary will result in an error.

The exe target does not use fmt functions anywhere and cannot depend on it.

[sergiud]

I cannot reproduce the problem at ad97258. Here's my MWE:

CMakeLists.txt

cmake_minimum_required (VERSION 3.0)
project (fmt-link)

set (CMAKE_CXX_VISIBILITY_PRESET hidden)

find_package (fmt REQUIRED)

add_library (mylib SHARED mylib.cpp)
target_link_libraries (mylib PRIVATE fmt::fmt)

add_executable (myexe myexe.cpp)
target_link_libraries (myexe PRIVATE mylib)

mylib.cpp:

#include <fmt/format.h>

__attribute__((visibility("default")))
std::string foo()
{
    return fmt::format("foo bar {}", 42);
}

myexe.cpp:

#include <string>
#include <iostream>

extern std::string foo();

int main()
{
    std::cout << foo() << std::endl;
}

[phprus]

CMakeLists.txt:

cmake_minimum_required (VERSION 3.9)
project (fmt-link)

set(CMAKE_CXX_STANDARD                  17)
set(CMAKE_VISIBILITY_INLINES_HIDDEN     TRUE)
set(CMAKE_CXX_VISIBILITY_PRESET         "hidden")

include(CheckIPOSupported)
check_ipo_supported()
set(CMAKE_INTERPROCEDURAL_OPTIMIZATION  TRUE)

set(BUILD_SHARED_LIBS           OFF)
add_subdirectory(fmt-master)
#set_property(TARGET fmt PROPERTY POSITION_INDEPENDENT_CODE ON)

add_library (mylib SHARED mylib.cpp)
target_link_libraries (mylib PRIVATE fmt::fmt)

add_executable (myexe myexe.cpp)
target_link_libraries (myexe PRIVATE mylib)

mylib.cpp:

#include <fmt/compile.h>

__attribute__((visibility("default")))
std::string foo()
{
    return fmt::format(FMT_COMPILE("foo bar {}"), 4242);
}

Build:

cmake -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_BUILD_TYPE=Release . && make

Error:

/usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/bin/ld: libmylib.so: неопределённая ссылка на «fmt::v7::detail::basic_data<void>::digits»

[phprus]

openSUSE Leap 15.2
gcc version 7.5.0 (SUSE Linux) ... and gcc-8 & 9

[sergiud]

Still no linker error. I copy pasted your modifications and used a local clone of fmtlib included via add_subdirectory instead of using find_package

I'm on ArchLinux with gcc version 11.1.0.

Edit: I missed the foo function modification. Now I'm getting the linker error as well.

[sergiud]

The culprit seems to be the combination libfmt.a + mylib.so + -flto. The sole modification to fix the issue that works for me is

diff --git a/include/fmt/format.h b/include/fmt/format.h
index 3f6ab357..71df6ced 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -902,7 +902,7 @@ template <typename T = void> struct basic_data {
   FMT_API static constexpr const char right_padding_shifts[] = {0, 31, 0, 1, 0};
 };
 
-#ifndef FMT_HEADER_ONLY
+#if !defined(FMT_HEADER_ONLY) && defined(FMT_EXPORT)
 // Required for -flto, -fivisibility=hidden and -shared to work
 extern template struct basic_data<void>;
 #endif

@phprus Can you verify this? FMT_API does not need to be changed.

[phprus]

I will check this change a little later today.

But anyway static libraries don't need visibility attribute. Therefore, I think it would be better to change the FMT_API as well.

[phprus]

!defined(FMT_HEADER_ONLY) && - this condition is not needed, because FMT_EXPORT is defined only on shared fmt.

FMT_EXPORT - is are private cmake definition
FMT_SHARED - is are interface cmake definition

In this case condition defined (FMT_EXPORT) is not needed and has been removed.

[sergiud]

Why did you remove FMT_EXTERN_TEMPLATE_API definition?

[phprus]

FMT_EXTERN_TEMPLATE_API was added in the partial incorrect commit 13e6529 and is not used anywhere.

[sergiud]

Right, "partial incorrect commit" 👎

FMT_EXTERN_TEMPLATE_API was used before. But you're correct to note that it is not necessary anymore.

[sergiud]

Could you also please add the combination to the test suite? This case is currently not being tested.

[phprus]

Test added.

[sergiud]

Great, thanks! LGTM

[phprus]

Don't merge now.
I am making a change to the test.

[alexezeder]

Note that you can transform this PR to Draft until you are ready.

[phprus]

Changes applied.
@vitaut review my PR please.

[vitaut]

Merged, thank you both!